pkgs.ankisyncd and the services.ankisyncd service have been switched from an old obsolete version of anki-sync-server to anki-sync-server-rs
The old version only implemented the old protocol compatible with the old 2.1.15 anki package (and similarly old android/external clients), while the new one only implements the new protocol which will work with the current nixos anki packages, ankidroid etc, so if you were using it with clients held back on purpose you will need to upgrade your clients.
The password database and anki data itself is compatible, so upgrading clients should just work, resyncing if required.